Chances are your system has AHCI enabled, and the MB manufacturer (thus "BIOS authors") chose not to implement tie-ins to AHCI for the BIOS interface bits.
The AHCI option ROM would be handling all the I/O to any SATA-attached drive, which explains how 1) you're able to boot off the hard disk, and 2) that Windows sees either device.
For example, I have a couple boards where the BIOS list "No device" for all SATA-attached drives when the controller is set to AHCI mode, while in IDE mode the BIOS lists all the attached drives. The AHCI option ROM device probe does show devices in the former case. Likewise, after POST but before booting, boot device selection changes as well depending on if AHCI or IDE mode is selected, especially for optical drives. (I can take a screenshot of this if you'd like, but it would probably just confuse you even more)
The motherboard manufacturer may have chosen to not display an AHCI device probe screen during option ROM load. That's their right/choice.
If all this truly bothers you, or the answers don't suffice (i.e. too speculative rather than factual), contact the motherboard manufacturer
. They are the only ones who will be able to tell you without a doubt if this is normal or not. My general opinion mirrors that of Dogg
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.